United States Patent Application Publications US 2003/0153768; US 2003/0050299 disclose compounds such as those shown below:

U.S. Pat. Nos. 5,945,442; 5,952,370; 6,030,995 and 6,051,598, which are all related to each other as being based on same or related disclosures, describe compounds which are structurally similar to the known compounds shown above.
A publication in Journal of Labelled Compounds & Radiopharmaceuticals (1996), 38 (3), 285-97 discloses the compound of the formula:

Patent application WO 081273 published on 3 Aug. 2006 also discloses compounds which are structurally related to the compounds shown above.